Abstract

Lineage zones and so-called datums based on phylogenetic events, particularly speciation events are currently very popular among biostratigraphers. The underlying assumption for proposing and using datums based on speciation events has to be that the species replacement, on a wide or global scale, was 'instantaneous' showing very little, if any, stratigraphic overlap in the geologic record. It is argued, and demonstrated, that unless it can be convincingly shown that the process of speciation involved was sympatric (phyletic) then such an assumption may be unwarranted and the proposed datum mis-leading. This conclusion is supported by demonstrating that the Pydogus serrus to P. anserinus species transition probably occurred according to an allopatric mode of speciation, leaving the ancestor surviving for an unknown length of time.